understanding anthropogenic (human-caused) sources of mortality in birds

A new compilation of papers in the journal Avian Conservation and Ecology points out newly-assessed pieces in the "big picture" regarding avian mortality caused by humans. These papers focus on how these factors affect bird species in Canada. But generalizations can be drawn, and the lead paper is especially enlightening for those of us who want to understand more about how these broad processes affect bird populations at larger scales. The following quote points out several facts of great interest to those concerned with the health of bird populations:

"Population size and status, timing of mortality, and temporal or intersexual variation in mortality can all affect the ability of populations to withstand additional mortality. Density-dependent natural survival is the putative mechanism driving the compensation, so populations at habitat carrying capacity should be more resilient to additional mortality than low density or declining populations (Nichols et al. 1984, Bartmann et al. 1992). Timing of the mortality event also may play a role; mortality is more likely to be additive when it occurs during or after periods of high natural mortality, and more likely to be compensatory when it precedes such periods (Kokko 2001). These generalizations suggest situations where mortality might be more or less likely to have significant effects on population status. Determining the actual demographic effects of such mortality, however, would require spatially explicit study of individual species. For most species, it is not possible to account for heterogeneity in survival between sexes and age classes, for dispersal, and for spatial structure in populations and mortality given the paucity of reliable data.

Dismissing human-related mortality as compensatory, even when populations are well monitored, is a risky bet. Researchers without adequate data can get it wrong. For example, the argument that the high mortality of American Black Duck from hunters was totally compensatory was wrong (Grandy 1983), as documented by Francis et al. (1998) who analyzed 44 years of banding data over three periods of increasingly restrictive harvest regulations and determined that estimated mean survival rates increased from the first to the second period consistent with a model of additivity of hunting mortality. Effects of some sources of mortality may take time to recognize, such as the slow-motion catastrophe for birds caused by second generation anticoagulant rodenticides (Thomas et al. 2011). Many sources of mortality also may work together cumulatively to suppress populations, and at different places across their life cycles (Loss et al. 2012). Assumptions about population-level effects may be misleading if analyzed at the national scale, which may mask effects that are significant at local levels but not evident at coarser spatial scales.

Perhaps more importantly, anthropogenic avian mortality affects more than just the species being killed. When considering effects of human activities on wildlife and ecosystems, the “legacy effects” of habitat loss and degradation are often the focus. Unnatural removal of birds from the environment, however, can still affect ecosystems even if habitat remains intact. If a cat kills a bird, the bird is lost as prey for a raptor (George 1974). If a bird dies from impact against a window and is swept away in the garbage, it cannot be food for its natural decomposers. When a bird is killed as a nestling by a mowing machine, it is not alive to eat insects for several months until it might have otherwise died of natural causes during migration (Whelan et al. 2008). Birds’ perception of hazards on the landscape can also have important effects on behavior, with indirect but significant adverse consequences (Bonnington et al. 2013). Disturbance and incidental mortality can alter timing of breeding, habitat use, and foraging behavior—all with the potential to influence ecosystems and ecosystem services. All of these nuances are lost when the focus is only on direct effects on single-species population dynamics." (These quotes are from "On Avian Mortality Associated with Human Activities" - Travis Longcore and Paul A. Smith. Avian Conservation and Ecology, 2013 Vol 8, No 2).  

Read more at this link:  http://www.ace-eco.org/vol8/iss2/art1/